Hi Tim--
I was on the team that built a couple Cob Hat projects. I can summarize a few things regarding the build at the Love Shack, as well as the Minnie Mouse (which is inside the Love Shack).
WHAT I KNOW ABOUT THE MINNIE MOUSE
The Minnie Mouse has a regular-size barrel (I think that's 24" across). The
wood feed is a tiny 4" across. The riser goes up to the typical height, but the chimney comes vertically out of the top of the barrel next to the riser. There is no typical mass outside the barrel. Instead, bricks were piled-up inside the barrel in an attempt to absorb heat and then slowly release it. The bricks were also placed to discourage radiant heat from extending into the nearby walls (they wanted the heat to radiate towards the occupant and/or the cabin space instead).
Important to note: if I recall correctly (so please understand that I may be misinformed on this), the chimney opening is close to the bottom of the barrel.
If you've ever seen or heard of a "hobo stove" or "hobo heater," this is a similar idea. [EDIT: in my quick
online search I've not found anything to match what I'm describing with the name "hobo
anything," so I will need to look into this more.]
Paul has also developed this into the "juice box
straw," found in the current Library build here at
Wheaton Labs. The point of having the chimney opening so low in the barrel is so that the hotter gases will linger, while the cooler gases are the ones that will be forced to go up the chimney, due to heat stratification.
Over time, we observed things similar to what you describe in your cabin with the
wood stove. Although it did retain heat, it didn't retain as much as we wanted. Furthermore to have a tolerable temperature in the morning hours, one would need to feed the heater for over an hour. In my
experience, this made the cabin too warm for my tastes, though I know of
Boots who, in the "pre-Cob-Hat" days, were able to increase the temperature to something in the 85 degree F range. That's waaaaay too hot for me, personally.
So we developed the cob hat through Paul's direction.
HOW TO BUILD A COB HAT:
- construct a "cob ring" that surrounds the top edge of the barrel and maybe 2" below the edge (so imagine cob "clamping" around the top edge and lip of the barrel). Inside this ring, we also used a few pieces of angle iron as a rudimentary rack for someone to heat or bake things without them resting on the top of the barrel itself.
- The ring ought to be 6" tall from the top surface of the barrel to the top of the ring, and open in the front, facing you (this is the "pizza slot"). The pizza slot opening will be roughly 5" tall, from the lip of the barrel edge to the top of the cob ring. Make the cob ring thick, as it will need to support the weight of the cob hat on top of it. I think the thickness of the rings we've made is 2" or maybe a little more than that.
- fill this ring area with wet sand, all the way up to the top edge of the ring. Imagine that the area you've filled with sand will be the hollow space underneath the cob hat once it's built.
- pile on the cob. Our mix, generally speaking, was heavy on sand and light on straw. You want a drier mix than typical, as it needs to be firm and almost sculpt-able. Shape it however you like, though our tests have been with roughly dome-shaped hats.
- wait a day or so, and observe the shape. Shore it up, reshape it, etc. if required.
- once things seem to have stayed in the shape you want, start firing the stove, for up to an hour at a time. This will help evaporate the moisture in the cob.
- once you've done this
enough to have an intact "shell" on the outside that's firm to the touch, scoop out the sand. We also finished this cleanout with a shop-vac and narrow vacuum nozzle.
- continue to do the hour-long burns, now that the sand is out of the way. Eventually, the cob will be entirely dry. We've seen hairline cracks in some of the cob hats (like the one on the Minnie Mouse), and this is likely due to having a cob mix with more
water than needed.
TROUBLESHOOTING AND OTHER DEVELOPMENTS:
-> As the cob hat dried over repeated firings, tiny hairline cracks appeared on the surface. So far as we have observed, this hasn't caused the cob hat to fall apart, fail, shoot out steam, or any other strange behaviours. They have not increased over time, over repeated, hot firings.
-> If you've found that your pizza slot is too narrow, simply take a mallet and chisel, and knock away the cob you don't want. Keep in mind however, that the cob ring is what supports the cob hat.
->In a different build of the cob hat, instead of having cob on the top surface of the pizza slot, we've added a large aluminum pizza pan as the "roof" of the opening. This hasn't been tested extensively, though in my opinion it has a serious future.
->We've also experimented with having a "sand core" inside the cob hat. Again, this needs some additional testing and observation (and maybe more explanation). It's only a matter of time before we have something to report about this and other developments.
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Hopefully the above has been helpful...! Personally, I think the cob hat is an effective, compact tactic for addressing both heat loss and heat absorption in small environments that let heat seep out too quickly.
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